[xavoy]

They are called Makyo in Zen (check wiki).

They are just another distraction, like any other thought but visual. Just let them go. The bigger a deal you make of them, the harder that will be. Acknowledge them and let them go.

As you progress in your practice you might have spacial and auditory hallucinations too. The same thing applies here. Just let them go.

[silentbicycle]

Yeah, it's just mental noise. Do you ever think you're hearing voices as you drift off to sleep, misread something you saw briefly as saying something very different, or mistake someone in a crowd for someone you knew?

Our brains constantly seek patterns in a sea of stimuli, and sometimes they create meaning out of noise. (http://en.wikipedia.org/wiki/Pareidolia)

"We are all greater artists than we realize." -Nietzsche

[mannicken]

If I relax and continue with hallucinations I start vibrating and having an out-of-body experience. It's fun and great but I like normal reality more :)